Microporous membranes are used in a wide variety of applications. Used as separating filters, they remove particles and bacteria from diverse solutions such as buffers and therapeutic containing solutions in the pharmaceutical industry, ultrapure aqueous and organic solvent solutions in microelectronics wafer making processes, removing bacteria from food and beverage products and for pre-treatment of water purification processes. In addition, they are used in medical diagnostic devices, where their high porosity results in advantageous absorption and wicking properties.
Microporous membranes have a continuous porous structure that extends throughout the membrane. Workers in the field consider the range of pore sizes to be from approximately 0.02 micron to approximately 10.0 microns.
Microporous membranes are described as symmetric or asymmetric. Symmetric membranes have a porous structure with a pore size distribution characterized by an average pore size that is substantially the same throughout the thickness of the membrane. In asymmetric membranes, the average pore size varies through the membrane, in general, increasing in size from one surface to the other. Other types of asymmetry are known. For example, those in which the pore size goes through a minimum pore size at a position within the thickness of the membrane.
Microporous membranes based on semi-crystalline and glassy polymers have been previously prepared. Most of the commercial membranes of these polymers are symmetric in nature. The production of such microporous membranes are described, for example, in U.S. Pat. Nos. 4,208,848 and 5,736,051 for PVDF and in U.S. Pat. Nos. 4,340,479, 4,707,266, 6,056,529, 4,770,777 and 5,215,662 for polyamide membranes. These preparations are generally described to consist of the following steps:
preparation of a specific and well controlled polymer solution, the casting solution, comprising a polymer, and a solvent system. In many cases two polymers are used, where one polymer is used to provide strength or other mechanical properties, and the second polymer is used to provide a functional benefit, such as hydrophilicity. The solvent system comprises a solvent and optionally, one or more additives, usually nonsolvents or swelling agents for the polymer or polymers;
casting, i.e., coating, a relatively thin layer of the polymer solution onto a temporary substrate;
immersing and coagulating the resulting film of the polymer solution in a nonsolvent;
removing the temporary substrate and
drying the resulting microporous membrane.
In some manufacturing processes, a permanent reinforcing web, such as a nonwoven fabric, is used as the substrate. Removal of the temporary substrate in this instance is not required, as the reinforcing substrate becomes part of the overall structure.
A similar process involving extruding the polymer solution through a fiber spinneret with a lumen is used to make hollow fiber membranes.
Polyvinylidene fluoride (PVDF) membranes as described above are made by casting a PVDF solution into a specific coagulant (methanol) which allows the formation of a microporous, symmetric membrane. A similar process is used for symmetric polyamide membranes. In these prior art processes, the semi-crystalline polymers used primarily lead to symmetric membranes.
Representative semi-crystalline polymers include polyvinylidene fluoride, polyvinyl fluoride, polyvinyl chloride, polyvinylidene chloride, polyamides, commonly known as nylons, such as nylon 6, nylon 6,6, nylon 6,12, and nylon 4; aromatic polyamides such as polyphenylene terepthlamide, and cellulose esters, such as cellulose acetate.
Membranes made from such semi-crystalline polymers have a characteristic property whereby the thermal history of the polymer solution prior to casting has a dramatic effect on membrane performance. In general terms, it has been found that the higher the maximum temperature (Tmax) to which the solution is heated to, the larger the rated pore size of the resulting microporous membrane. In microporous membranes, pore size is related to the easily measured bubblepoint.
Pore size usually-refers to the ability of a porous media or membrane to filter material larger than a specified size from a fluid (be it gas or liquid). Put another way, pore size refers to a measure of the size of the passageways available for fluid passage or a measure of the diameter of the pores in a porous media or membrane. As a simple example, a membrane rated as “0.1 microns” would retain all material larger than 0.1 microns and pass all other material, including the solvent or fluid carrier that are smaller than 0.1 micron. Pore diameter can be described in by a variety of methods. Pore diameter can be specified as the diameter of the smallest particle or molecule that is retained by the porous media or membrane. When the porous structure of a porous media or porous membrane is analyzed by microscopy, pore diameter can be described as the diameter of the largest circle that can be inscribed in a pore, or by a hydraulic diameter, defined as one fourth the area of a pore divided by its circumference.
Pore size can be specified by a number of methods. Bubble point methods use the fact that to expel the liquid in a liquid-filled capillary requires a gas pressure dependent on capillary diameter to expel the liquid. The bubble point of a membrane is measured by applying a gas pressure to a liquid saturated membrane and gradually raising the gas pressure until the first gas bubble or stream of bubbles is observed rising from the side opposite that of the gas application side. The pressure at which the first bubbles appear on the opposite surface of the membrane is related to the largest pore size of that membrane. Bubble point is inversely related to pore size, with higher bubble points signifying smaller pore size of the membrane.
Filtration of various particles can be used to determine pore size. Colloidal gold particles, polystyrene latex particles, bacteria, viruses and dendrimers have all been used. The subject porous membrane is tested with various particles of known size, and the smallest particle that will not pass is used to give a pore size to the membrane. In another variation of such testing, a preset retention percentage of a particle of known size is used to rate pore size.
Soluble species can be used as well. The diameter of a soluble macromolecule is primarily dependent on molecular weight, and also on solution conditions. This diameter can be used to determine a pore size measure of porous media or membranes. Proteins, soluble polymers such as dextrans (polysaccharides), polyethylene oxides, and polyvinylpyrrolidone have been used for this purpose.
Microscopy can be used, with or without computerized image analysis, including scanning electron microscopy, transmission electron microscopy, and atomic force microscopy.
Scattering methods can be used. Light scattering, acoustic scattering, and neutron scattering methods have been described as methods to determine a measure of pore size.
Membrane manufacturers want to control pore size for reasons of product uniformity. For critical applications, such as sterile filtration, pore size cannot fall below a set minimum or above a set maximum and still have the desired retention and validation properties. Typically, one strives to form a product with a relatively small pore size distribution. The ability to rapidly adjust membrane properties during a continuous manufacturing process would allow manufacturers to bring their product within a narrower range of the desired properties.
In one method of controlling pore size, the polymer solution is made at a relatively low temperature in a typical manufacturing stirred tank vessel or similar device and then heated to the desired maximum temperature by, for example, a heated jacket. Practitioners skilled in the art are well aware that poor control over the final temperature, or maldistribution of temperature in a mixing vessel will result in membranes having bubble points that deviate from the desired bubble point. For this reason, close control of the final solution temperature, and the uniformity of the final temperature throughout the solution volume are highly desired.
Inconsistency in casting solution history can therefore cause reduced process yields. It can be appreciated that fine control over the thermal history of a large mass of viscous solution is difficult. As an alternative, in-line heating and cooling treatment is sometimes used in order to provide improved control over the thermal history of the polymer solution being processed. An in-line process provides a means for heating the solution as it is transported through a pipeline, thereby reducing the effective mass of solution being heated. The shorter heating contact time necessitated by in-line heating requires uniform local mixing to obtain even heat treatment and sophisticated process control to insure consistency of time and temperature during the entire casting process.
U.S. Pat. No. 6,056,529 describes a method using an inline heating system to heat a portion of a single batch solution being transported before casting a membrane. This method is limited to a single batch and is directed to reheating a portion of a prepared solution, with all the complications involved in closely controlling temperature.
These methods use fine control of polymer solution temperature to control membrane pore size. This requires bringing the polymer solution uniformly to within a few tenths of a degree of the desired temperature needed to attain the specified bubble point or pore size. These methods are limited to cases with solutions containing semi-crystalline polymers.
In contrast, the invention described herein can be used with any set of similar polymer solutions, chosen to bracket the desired final properties.
At least two solutions are used in the process of the present invention. The resulting pore size, or bubblepoint of each solution is known before full casting is done. Methods for this are explained in the Detailed Description. The inherent properties of the individual solutions are not changed, as by heating, prior to casting.
Furthermore, increasing temperature will tend to cause gas bubbles, due to the lower solubility of gases in liquids at higher temperatures. Gas bubbles are a source of defects in cast membranes and have to be removed before casting, adding to process complexity.
In a manufacturing situation, batches of polymer solution destined for membrane casting are sometimes made which are not exactly of the bubble point desired. This is usually determined by casting a test membrane from a portion of the solution batch. Rather than disposing of such batches, it would be economically useful, as well as environmentally beneficial, to be able to use such “off spec” batches. In the invention to be described, such batches can be made useful by selective in-line blending with a properly selected second batch to obtain the desired membrane pore size.